This invention is directed to an improved floatation bath process for separating certain molded thermoplastic parts from other materials, including other plastic parts. Such other materials being separated may include dirt, glass, metal, different thermoplastics or thermoset plastics. Such molded thermoplastic parts that may be separated from each other or other materials include plastic parts molded from polyethylene terephthalate, polybutylene terephthalate, aromatic polycarbonates, polyvinyl chloride, etc.
Currently, to separate two or more materials with specific gravities above 1.0 by the floatation bath process, the industry generally uses halide salts such as, for example, sodium chloride (NaCl) or calcium chloride (CaCl.sub.2) in order to increase the specific gravity of the water medium in the float bath to a specific gravity such that it lies between the specific gravity of the materials being separated. That material having a lighter specific gravity will float to the top of the bath and that material having a specific gravity greater than the specific gravity of the floatation medium will sink to the bottom of the floatation vessel. Thus materials may be separated from each other by the floatation process. However, there are disadvantages with the use of halide salts such as NaCl or CaCl.sub.2 in floatation separation baths. For example, with NaCl, the specific gravity of the water solution in the bath can only be increased to about 1.2 grams/cubic centimeter (g/cc). With CaCl.sub.2, the specific gravity of the water in the bath can only be increased to about 1.4 g/cc. Also, dissolving CaCl.sub.2 in water is an exothermic reaction, thereby making it difficult to control the temperature of the float bath. In addition, with the separation of certain plastics, residual halogen ions from halide salts can attack the polymer chains of some plastics. For example, with a polycarbonate, the carbonate polymer chains can be attacked by halogen ions and thus can affect a decrease in the molecular weight of the polymer. Further, halogen ions can corrode metal, which would reduce the service life of the floatation tanks, as well as extrusion or injection molding equipment when the recycled plastic is melt processed and molded.